Recently, recording of higher picture quality and higher tone quality is required for a videotape. In order to cope with this, a tape is now being employed on which is evaporated a Co metal or a Co--Ni alloy capable of recording and having higher magnetic density as compared with a conventional tape coated with ferrite or a metal. The Co metal or the Co--Ni alloy is excellent in magnetic properties, i.e., coercive force and residual magnetic flux density. Therefore, recording of high picture quality and high tone quality is enabled by improving the magnetic density in a tape on which a Co metal or a Co--Ni alloy is evaporated. Further, future demand increase is expected for such an evaporated tape in relation to long-time recording, application to high-definition broadcasting and the like.
With reference to FIG. 2, an evaporation step for such a tape is described. FIG. 2 is a schematic diagram showing an evaporation unit A delivery shaft 13, a take-up shaft 14, a crucible 16, an electron gun 17 and a feeder 18 are arranged in the interior of a vacuum chamber 11. The delivery shaft 13 delivers a base film (polyester film, for example) 12, and the take-up shaft 14 takes up the evaporated base film 12. The crucible 16 stores an evaporation material 15. The evaporation material 15 is heated, melted and vaporized by an electron beam which is emitted from the electron gun 17. The evaporation material 15 is fed from the feeder 18 to the crucible 16. In such a unit, the interior of the vacuum chamber 11 is held in a vacuum state of about 10.sup.-5 to 10.sup.-6 Torr, and thereafter the evaporation material 15 is heated by the electron beam to a temperature of about 2000.degree. C., to be melted and vaporized. The vaporized evaporation material 15 reaches an upper portion the base film 12. Thus, a thin film is formed on the base film 12 by evaporation.
A Co--Cr group alloy is also excellent in magnetic properties, i.e., coercive force and residual magnetic flux density. Therefore, the Co--Cr group alloy is employed as a material for a vertical magnetic recording thin film in recent years. This material is formed and employed as a vertical magnetic film by evaporation.
An evaporation method therefor is carried out in the interior of a vacuum chamber which is held in a vacuum state of about 10.sup.-5 to 10.sup.-6 Torr. An evaporation material stored in a crucible is heated by an electron beam to a temperature of about 2000.degree. C., to be melted and vaporized. The vaporized material is made to reach a surface of a substrate, so that a thin film is formed on the surface of the substrate by evaporation.
It is necessary to refill the evaporation material of a Co metal, a Co--Ni alloy or a Co--Cr alloy for the vaporized part. Such refilling is carried out through a pellet member of about 10 mm.phi. by 10 to 30 mm in size or a bar member of 30 to 80 mm.phi.. In general, a pellet member is dropped into a molten metal which is contained in a crucible, while a bar member is partially melted and dropped into a molten metal for refilling.
A method of manufacturing a pellet member is now described. First, a material such as a Co--Ni alloy is vacuum-fused, cast, hot-forged, and rolled to prepare a rolled wire. Then, this rolled wire is inserted in a hole of a die assembly as shown in FIG. 33, and a wire 9 which is projected from an opening portion of the hole is cut by shearing of a die A and another die B. Then, the as-obtained short member is subjected to barrel polishing and surface cleaning with an organic solvent, to obtain a pellet.
According to the aforementioned feeding method, however, evaporation conditions are instabilized following dropping of the pellet, to cause disturbance on a molten metal level of the evaporation material, scattering of the molten metal, non-uniformity of temperature distribution in the molten metal and the like. These instabilize the vaporization direction and the amount of vaporization of the material, and hence it has been impossible to manufacture an evaporated tape of stable quality.
On the other hand, it may be conceivable to feed an evaporation material by along wire, as a countermeasure for such a problem. It may be conceivable to continuously feed a long wire into a crucible to stabilize evaporation conditions, thereby manufacturing a highly reliable evaporated tape. In this case, a long-time continuous evaporating operation is advantageously enabled. Therefore, working of an evaporation material such as a Co--Ni alloy into a wire is desired. Among evaporation materials, that which is easy to elongate, such as aluminum (Al), is continuously fed by a wire in practice.
However, it is extremely difficult to wire-work a material such as a Co--Ni alloy. Namely, it is difficult to elongate such a material by cold working since the same is hard to work, and even if the same can be elongated by hot working, it is so difficult to completely round the wire surface that irregularity may be caused on the wire surface or the wire may be broken during feeding.
As shown in Japanese Patent Laying-Open No. 59-64734, further, there is a method of adding iron (Fe) into a Co--Ni alloy, thereby improving workability and toughness of the wire. In such a technique, however, there arises another problem such that excellent magnetic properties of the Co--Ni alloy are reduced. Namely, deterioration of excellent magnetic properties characterizing the Co--Ni group alloy is unavoidable since Fe is contained by 2 to 10 weight %.
As shown in Japanese Patent Laying-Open No. 3-236435, further, there is a technique of limiting impurities in an alloy, thereby improving toughness and the like. In this technique, however, only the alloy composition is studied and it is impossible to attain sufficient improvement of workability in drawing or the like merely by suppressing contents of oxygen, nitrogen and sulfur which are impurity elements.
On the other hand, two important points are present in a method of manufacturing a pellet member. One point is to reduce diametral deviation, and another point is to remove impurities (impurities, oxides etc. adhering in a previous step of hot forging, rolling or the like) from the surface. First, a problem caused by diametral deviation is described. Wile a rolled wire having large diametral deviation is cut in the aforementioned method of manufacturing a pellet, a sufficient clearance must be provided between the hole diameter of the die assembly and the diameter of the wire so that the wire passes through the hole of the die. When the rolled wire is cut in a state having large diametral deviation, the metal mold may be clogged with the wire. Further, the diametral deviation may be extremely increased or only a pellet member having a burr is obtained, leading to the problem that a feed pipe is clogged during feeding to an evaporation unit.
As to removal of surface impurities, on the other hand, it is impossible to sufficiently remove those deeply penetrating into the material, such as deposits in rolling, by the aforementioned barrel polishing and cleaning with an organic solvent. When impurities remain, the remaining impurities are vaporized with the evaporation material, and hence magnetic properties of the as-obtained tape are deteriorated. In order to further remove the impurities, therefore, it is necessary to carry out grinding with a lathe, or centerless grinding. However, cutting oil in a lathe or adhesion of new impurities such as abrasive grains in centerless grinding causes a problem, while the manufacturing cost is also increased. In such working, further, the material yield is reduced since the material itself is ground, leading to a subject of how to reduce grinding allowance.
When feeding by a pellet member is carried out, evaporation conditions are instabilized following dropping of the pellet, to cause problems of disturbance on a molten metal level of the evaporation material, scattering of the molten metal, non-uniformity in temperature distribution in the molten metal and the like. These instabilize the vaporization direction and the amount of vaporization of the material, to hinder quality stabilization in tape manufacturing.
In order to cope with such problems, it may be conceivable to work the evaporation material into a long wire and continuously feed the same into a crucible for stabilizing evaporation conditions, thereby manufacturing a highly reliable tape, as already described. In this case, a long-time continuous evaporating operation is also advantageously enabled, and hence working of a material such as a Co--Ni alloy into a wire has been desired.
Also in this case, however, minimization of the aforementioned diametral deviation and removal of surface impurities arise problems.